1. Field
The present disclosure relates to a light-emitting device, examples of which are a light-emitting diode (LED) and a laser diode (LD), and more particularly, to a reflective surface sub-assembly for such a device.
2. Description of Related Technology
Without any loss of generality, merely to avoid undue repetitiveness of the disclosure, the state of related technology is explained using an LED as a typical example of a light-emitting device.
The principle of operation of LEDs is based on a property of a semiconductor p-n junction. When a p-n junction is forward biased, i.e., a positive voltage is applied on the p-type semiconductor and a negative voltage is applied on the n-type semiconductor, charge carriers—electrons and holes—flow into the junction. When an electron collides with a hole, the electron recombines with the hole and because the electron falls into a lower energy level, the excess energy—equal to the difference between the electron and hole energy levels involved—is released in the form of a photon. This effect is called electroluminescence and the color of the light is determined by the energy gap of the semiconductor.
Because the collision between the electron and the hole is statistical in nature, the photons will be emitted from the p-n junction in random directions. To improve light extraction from the p-n junction of the light-emitting device, a layer of reflective material is applied to surfaces that are transparent to the emitted wavelength or have poor reflectivity of the emitted wavelength in an undesirable direction of emission. Reflectivity is characterized by a ratio of reflected to incident light. To achieve the best possible luminous efficiency, material with high reflectivity, e.g., noble metals like Pt, Au, Ag, or other materials, like Al, are used for this purpose. However, use of some of the materials, e.g., Pt, Au, both from the cost of the materials and the process of applying the materials to the surfaces, are relatively expensive. Other materials, e.g., Al, although relatively inexpensive, require expensive polishing and always oxides in an oxygen present environment. However, even after extensive—and expensive—polishing Al's reflective power is inferior to polished Ag. Additionally, although polishing increases total reflectivity, it does so by increasing specular reflectivity. However, as discussed in greater detail below, the material should possess diffusive rather than specular reflectivity. Ag, preferable to Al due to its superior reflectivity, is prone to oxidation and/or tarnishing, especially when the Ag surface comes in contact with some of the chemicals used in light-emitting device manufacturing process. An example of such would be the use of phosphors for light conversion in LEDs. Furthermore, Ag coated substrates require utter moisture protection, which increases the overall cost of an LED package, yet not always results in the desired level of performance and/or stability, i.e., resistance to change of parameters over time, of the device. For example, although all appropriate precautions have been taken during the manufacturing process, due to the fact that a case of the light-emitting device is not hermetically sealed against the environment in which the light-emitting apparatus is deployed, oxidation may happen over time, thus causing deterioration of a reflectivity, uniformity of the light and other parameters over time.
Accordingly, there is a need in the art for improvements in light-emitting devices to increase light output, simplify manufacturing process, decrease cost, and provide additional advantages evident to a person skilled in the art.